1. Field of the Invention
This invention relates to telephone systems and more particularly to a tone ringer for use in a telephone subscriber's subset.
2. Description of the Prior Art
In the past most telephone subset ringers have been of the electromechanical type. These units typically consist of an electromagnet that operates in response to ringing signals received from the telephone central office. In response to these signals a clapper or hammer type of mechanism is caused to periodically make contact with and then release, against a bell or gong type of mechanism. The resulting sound being transmitted to the subscriber to act as a signal that an incoming call is being received. Such electromechanical units are characterized by substantially large size and consequently are difficult to include in present day telephone subsets, which are substantially reduced in size over earlier versions.
Like most mechanical devices electromechanical ringers in continued operation are highly prone to changes in adjustment. Such changes frequently cause changes in tone, volume, etc. rendering the output signal unacceptable to the telephone subscriber. To correct such adjustments service calls are required with the obvious economic disadvantage. Also the output signals of electromechanical ringers are frequently mistaken for similar devices such as doorbells, etc. that are often located in an environment similar to that where a conventional telephone substation is employed.
The characteristic sounds produced by electromechanical ringers contain many high frequency components. Older subscribers may traditionally have a loss of hearing particularly at high frequencies and accordingly they experience considerable difficulty in hearing conventional electromechanical ringers.
More recently ringers of electronic construction have been developed. Some of these units have generated single frequency tone signals which have found a general lack of acceptance in public usage. In attempts to correct this fault in early electronic tone ringers various techniques have been developed for providing multitone signals. An early approach to the problem was to modulate the tone source with the incoming ringing signals, providing a composite signal whose characteristics were determined by a base frequency modulated by the frequency of the alternating current ringing signal received from the central office. Such signals were also found to be essentially unacceptable by telephone subscribers because the ringing frequencies utilized by telephone companies are many and varied. Some of the resultant output signals are not easily recognized as being indicative of incoming telephone calls. Accordingly subscriber acceptance was also poor.
Prior art electronic ringers have also suffered from other problems such as changes in characteristics due to the subscriber's line loop length and the use of multiple ringers on the same telephone line.
Early electronic tone ringers generally proved to be insufficient in output providing less than adequate signal strength. Several different techniques have been employed to correct this obvious defect, including the use of horns, resonators, etc., to acoustically improve the output signal of the tone ringer.
Many prior art electronic tone ringers also suffered from an inability to distinguish from spurious line signals and ringing signals, accordingly producing false tones that were not actually indicative of an incoming call.
An electronic tone ringer that eliminated most of the prior art problems, was disclosed in U.S. Pat. application, Ser. No. 385,134 filed Aug. 2, 1973 by Richard A. Morstadt. While essentially satisfactory in operation the Morstadt design under certain conditions was subject to so called "bell topping" or "chirping", a condition which causes the ringer to momentarily operate during dialing. Accordingly the present invention discloses an electronic tone ringer which may be considered as an improvement over the Morstadt design in that it provides all the features included in the Morstadt design and yet eliminates so called "bell tapping" or "chirping". The above referenced ringer was also of the straight line type and unable to function satisfactorily on a divided party line because of resulting heavy ground currents with possible unbalance and noise. Accordingly it is a further object of the present invention to provide an electronic tone ringer that is adaptable for two party divided ringing.